It all depends on the type of job. Each site will have different issues to deal with. How is the access, what type of block are you using, how high is the wall, do you need grid behind the wall.

All of these things need to be accounted for and will differ from job to job. Retaining walls can be one of the hardest things to price. After doing a couple you will figure out your things you might have missed in your pricing. But I would advise against just giving a square foot price.

-How many square feet of block will be used for base course? (It takes a lot longer to install these blocks than the rest of the wall)
-How many square feet of block will be used for the rest of the wall?
-How much material needs to be excavated for the wall, how long will it ake to move it on site or to remove it from site?
-If geogrid is needed how much additional soil needs to be excavated and reinstalled?
-Time and materials for drain pipe and gravel behind the wall.
-How long will it take to get the block from truck to wall? will it be moved there by hand or machine?
-Is block cutting required? (more labor)
-Will there be lawn repair?
-Is the customer indecisive, likes to talk your ear off, extremely picky? Charge more.